Aims of the study

Brendgen et al. (2005) conducted their research to explore the underlying causes of aggression in children, focusing on how genes and environment contribute differently to various types. The researchers aimed to determine the genetic and environmental influences (both shared and non-shared) on physical and social aggression in six-year-old twins. They also investigated whether physical and social aggression share the same causes and if one type might lead to the other.

Participants in the study

The study involved 234 pairs of six-year-old twins from Canada, who were already part of a larger longitudinal study. The twins were born between November 1995 and July 1998 and were assessed multiple times, but the focus was on data collected at around 73 months (just over six years old) to evaluate their social adaptation in kindergarten.

Breakdown of participants:

• Monozygotic (MZ) twins - 94 pairs (44 male and 50 female).
• Dizygotic (DZ) same-sex twins - 73 pairs (41 male and 32 female).
• Dizygotic (DZ) mixed-sex twins - 67 pairs.

Written consent was obtained from parents of all children in the participating classes before the study began.

Procedure for assessing aggression

To gather data on aggression, the researchers collected ratings from two sources who knew the twins well: their teachers and their classmates.

Teacher ratings

• Teachers completed two questionnaires for each twin.
• For social aggression, they rated descriptions such as "tries to make other children dislike a child" or "becomes friends with a child for revenge."
• For physical aggression, they used a three-point scale to answer questions like "Does the child get into fights?", "Does the child hit?", or "Does the child bite?"

Peer ratings

• Classmates were given pictures of their peers, and researchers first confirmed they could identify everyone.
• Each child individually circled three pictures that matched behavioural descriptions, without discussing it with others.
• For social aggression, descriptions included "tries to make others dislike a child" or "tells mean secrets about others."
• For physical aggression, descriptions were "hits others," "bites others," or "gets into fights."
• Nominations were tallied to give each twin a total score for social and physical aggression.
• Notably, 82% of twins received at least one nomination for social aggression, and 62% for physical aggression.

Results from teacher and peer ratings

The study revealed differences in how aggression was perceived and its potential causes. There was also a moderate correlation between peer ratings and teacher ratings.

Key findings on gender and aggression types

• Teachers rated boys as more physically aggressive than girls, and girls as more socially aggressive.
• Peers rated boys as more aggressive in both physical and social forms.
• Physical aggression appeared to lead to social aggression, based on peer observations.
• MZ twins had twice the correlation for physical aggression compared to DZ twins, indicating a biological basis.
• Correlations for social aggression in MZ and DZ twins were very similar suggesting environmental causes.

Variance in aggression types

Variance analysis showed the percentage contributions of genetic, shared environmental, and non-shared environmental factors.

These results highlight that physical aggression has a stronger genetic component, while social aggression is more influenced by non-shared environments.

Conclusions and implications

Brendgen et al. (2005) concluded that physical aggression is largely influenced by genetic factors. In contrast, social aggression is primarily shaped by environmental factors. This suggests physical aggression may be inherited, while social aggression develops through experiences. The study also found that physically aggressive children were more likely to engage in social aggression, possibly due to an interaction between genes and environment - for example, a genetic predisposition to aggression might be channelled into social forms depending on surroundings.